Due to an attractive balance of performance and cost, polyolefin resins, such as polyethylenes, polypropylenes, copolymers of ethylene and propylene, and compositions based thereon, are widely used in coating and insulation applications. These applications include: heat-shrinkable corrosion-protection sleeves for oil and gas pipeline joints; solid and foamed coatings for the corrosion, mechanical and thermal protection of pipelines and pipeline structures; wire and cable insulations and jacketing; and heat-shrinkable extruded tubing or molded shapes for the electrical insulation and mechanical protection of wires, cables, connectors, splices and terminations.
Many of these applications require that the coating or insulating material provide acceptable thermal and mechanical performance at temperatures close to or above the softening or melting point of the thermoplastic polyolefin resin(s) from which it is made. Such performance requirements include, but are not limited to, long-term continuous operating temperature, hot deformation resistance, hot set temperature, chemical resistance, tensile strength and impact resistance. To achieve these requirements it is necessary to impart some thermoset characteristic to the resin or polymer. This is accomplished by crosslinking the molecular structure of the polymer to some required degree. Crosslinking renders the material resistant to melting and flowing when it is heated to a temperature close to or above the crystalline melting point of the highest melting point polymeric component of the composition. This property is also necessary for the production of heat-shrinkable articles, such as pipe joint protection sleeves, where crosslinking imparts controlled shrinkage, or heat recovery, characteristics, and prevents the material from melting when it is heated to the temperature necessary to effect heat recovery.
Crosslinking, or curing, of polyolefin-based coatings or insulating materials is typically accomplished through one of two basic methods: by irradiation, such as exposure to electron beam radiation; or by thermo-chemical reaction, such as that induced by peroxide decomposition or silane condensation. The advantages and disadvantages of these methods are noted below.
Irradiation of the polymer by electron beam generates free-radicals on the polymer chains which then covalently combine to effect crosslinking of the polymer. It is an instantaneous and clean method, but requires expensive, and potentially dangerous, high voltage “electron-beam” equipment. It also has limitations in terms of the product thickness and configuration that can be crosslinked uniformly.
Peroxide crosslinking is also a free-radical process but here the free-radicals are chemically generated through decomposition of the peroxide by heat. The process is thickness independent but needs substantial amounts of heat to effect crosslinking, is performed at relatively low processing speeds, and is frequently coupled with cumbersome and expensive processing equipment, such as pressurized steam or hot-gas caternary lines. A major disadvantage of using the high temperatures required to induce peroxide crosslinking (typically 200 to 350° C.) is potential softening, damage, and oxidative degradation of the polymer.
Silane crosslinking, also known as moisture crosslinking, occurs via hydrolysis and condensation of silane functionality attached to the polymer to be crosslinked. It is a relatively inexpensive process but requires a preliminary silane grafting or copolymerization operation, has restrictions in terms of polymer formulation flexibility, and is very time dependent, requiring many hours or days in a hot, moist environment to achieve full crosslinking of the polymer.
Typically, the crosslinking operations described above are performed as separate and discrete processes subsequent to melt processing, or forming, of the polymer article. It is, however, advantageous in terms of production efficiency, product throughput, and operating cost to perform the crosslinking operation at the same time as, and on-line with, the polymer processing, or forming, operation, and immediately following solidification of the formed article.
Of the methods described above, only the peroxide method realistically provides the opportunity of crosslinking in situ or “on-line” with the polymer processing or forming operation. The size, complexity, and safety risks of an electron beam typically preclude its use as an on-line crosslinking device. In the case of silane crosslinking, the crosslinking reaction can only be accomplished off-line since it is a highly time-dependant reaction, influenced by the diffusion of moisture into the polymer.
Crosslinking using ultra-violet (UV) radiation, namely radiation in the range of 200 to 500 nanometers wavelength, and also known as photo-crosslinking, provides a potential solution to the problems described. Compared with electron beam irradiation, the UV source required to effect crosslinking is relatively small, more easily configurable, less expensive and safer to use. It offers the potential of a portable crosslinking device which can be moved into position downstream of the polymer melt processing, or forming, operation. For example, the device may be positioned between an extruder and a product handling, or wind-up, station of a continuous polymer extrusion process, to allow on-line crosslinking of an extruded article, such as sheet, tubing, or wire insulation.
There are two primary methods of crosslinking or polymerization using UV radiation: free-radical and ionic.
UV free-radical crosslinking results from a reaction involving a photoinitiator, such as benzophenone, benzyldimethylketal and acylphosphine oxides, which absorbs UV light to dissociate into free radicals which can then initiate the crosslinking or polymerization reaction. A multifunctional crosslinking agent, such as triallyl cyanurate or trimethylolpropane triacrylate, may be additionally incorporated to achieve higher levels of crosslinking.
Unfortunately, a major disadvantage of UV free-radical crosslinking has been that it cannot readily be used for crosslinking thick or solid polymer sections, such as the functional thicknesses required for the pipe coatings, heat-shrinkable coverings, and wire and cable insulations described above. This is because of the relatively weak intensity of UV light which results in poor penetration of the radiation through the solid material, compared with electron beam radiation, for example. This is particularly the case with semi-crystalline polymeric materials, such as polyolefins, where the dense crystalline regions are relatively impenetrable to UV radiation. The effectiveness of UV free-radical crosslinking is also compromised if the resin to be crosslinked comprises additional materials such as filler and stabilizer additives, since these can provide further barriers to penetration by the UV light as well as interfering with the crosslinking reaction by neutralizing the free-radicals required for crosslinking. In addition, UV free-radical crosslinking is severely inhibited by the presence of oxygen, and for this reason is ideally performed in an inert atmosphere, such as nitrogen.
Traditionally, therefore, the use of UV free-radical crosslinking has been restricted to the curing or polymerization of liquid or low viscosity functional monomers or oligomers, such as acrylates, methacrylates and unsaturated polyesters, in thin (typically less than 0.250 mm., more typically less than 0.100 mm.) coating applications, such as film coatings, paints, inks, and varnishes, or for sealants and pressure sensitive adhesives, whereby the liquid or low viscosity monomers or oligomers are converted to a solid or gel-like material.
UV crosslinking by ionic reaction, that is anionic or cationic polymerization, and more particularly cationic polymerization, has historically found limited use compared with the UV free-radical process due to the unavailability of effective cationic photoinitiators. However, recent technical advances in cationic photochemistry are now making this technique more attractive for commercial applications. The process relies on the cationic polymerization of epoxy, oxetane, vinyl ether and similar functionalities by strong protonic acids created by the UV irradiation of onium salts, such as aryldiazonium salts, triarylsulphonium and diaryliodonium salts, for example. The first type generates Lewis acids whilst the last two types produce Bronsted acids, these being preferable as initiating entities for cationic polymerization.
A very useful feature of cationic polymerization is that the reaction is mostly thickness independent and will continue to proceed to completion “in the dark” after the UV source has been removed. In addition, the cationic photoinitiation reaction is not inhibited by oxygen as is free-radical photoinitiation.
An example of a typical cationic reaction mechanism is shown below in relation to the polymerization of a cycloaliphatic epoxide.
Reaction Step 1: On UV irradiation, the cationic photoinitiator interacts with active hydrogen naturally present to produce a strong protonic,
or Bronsted, acid, and various aryl sulphur compounds:
Reaction Step 2: The acid will protonate epoxy, or oxirane, groups, and polymerization then proceeds by ring-opening reaction:

European Patent 0490854A2 describes one attempt to address the problem of crosslinking relatively thick extruded polyethylene materials by UV radiation (in this case an extruded strip of thickness 0.5 mm.). A proprietary benzophenone free-radical photoinitiator having low vapour pressure and high polymer solubility is used in combination with a crosslinking promoter to effect rapid crosslinking of extruded polyethylene. However, due to the problems associated with UV penetration described earlier, the crosslinking operation needs to be carried out in the melt, in other words before the polymer has solidified or crystallized. This severely restricts the use of this method in most extrusion operations, where it is necessary to shape the polymer and cool the material below its melting point immediately after exiting the extruder die. Crosslinking of the polymer in the melt state necessarily fixes the shape of the extrudate or dramatically increases the material viscosity, thereby limiting any downstream sizing or shaping operations that may need to be performed. It is also practically very difficult to insert a UV radiation device between the extruder die and adjacent cooling equipment, such as a water trough or casting stack, without severely impeding the overall extrusion operation.
Japanese Patent Application 05024109A2 uses a similar free-radical technique to crosslink an extruded polyolefin tube which is then expanded to create a heat-shrinkable tubular product. Again this process is performed in the melt state, so the limitations described above remain unaddressed.